This invention relates to methods for the manufacture of polyalkylene terephthalate resin compositions. In particular, a polyalkylene terephthalate resin such as poly(butylene terephthalate) is prepared employing a titanium-containing catalyst complex comprising a phosphorous, nitrogen, or boron atom.
Polybutylene terephthalate (PBT) resin is a well-known polyalkylene terephthalate that is semi-crystalline and has several desirable properties alone or in blends. Compared to amorphous resins such as acrylonitrile butadiene styrene (ABS), polycarbonate, and polystyrene, a semi-crystalline resin like PBT can show higher solvent resistance, strength, and stiffness due to the presence of crystalline spherulites in the resin. PBT resin is used in many applications in which its solvent resistance, strength, rigidity, and lubricity are needed, most commonly in durable goods that are formed by injection molding. Applications include electronic and communications equipment, computers, televisions, kitchen and household appliances, industrial equipment, lighting systems, gardening and agricultural equipment, pumps, medical devices, food handling systems, handles, power and hand tools, bobbins and spindles, and automotive parts in both under-the-hood and exterior applications. Additionally, PBT is widely used to form electrical connectors. Through its many blended products PBT can be tailored to a wide variety of applications.
It is generally known that PBT can be made by reacting 1,4-butanediol (BDO) with terephthalic acid (TPA) or dimethyl terephthalate (DMT) in the presence of a transesterification catalyst. U.S. Pat. Nos. 7,129,301; 6,020,393; 4,328,059, and US Patent Pub. 2005/0113534 A1 disclose various catalysts for the polymerization of polyesters.
Commonly used catalysts for the polymerization of PBT include tetraalkyl titanates. Among the various titanates are tetraisopropyl titanate, tetrabutyl titanate, and tetra(2-ethylhexyl) titanate. For example, JP 60147430 discloses a method of producing polyester by esterifying terephthalic acid, adipic acid and 1,4-butanediol in the presence of a titanium compound and a pentavalent phosphorus compound. U.S. Pat. No. 6,303,738 discloses a process for producing copolyester containing adipic acid in the presence of TYZOR® IAM (available from DuPont), which is prepared through the combination of tetraisopropyl titanate (TPT) and a mixture of butyl phosphate and dibutyl phosphate.
The catalyst used to prepare PBT is not typically quenched (deactivated) at the end of the polymerization process. An active catalyst in the resin, however, can sometimes lead to undesirable reaction of the PBT in subsequent processing. Blends containing polyalkylene terephthalates such as PBT, on exposure to high temperature and humidity, can exhibit hydrolytic degradation, especially under caustic conditions. Another problem associated with some blends is transesterification, which can lead to loss of mechanical properties. Catalyst quenchers can be added to thermoplastic compositions that comprise the polyalkylene terephthalate resin to prevent such transesterification, but such quenchers can also promote degradation of polymer chains and contribute to decrease in hydrolytic stability. Quenchers have included conventional phosphorous derivatives such as phosphoric acid and phosphates. Phosphite stabilizers are less satisfactory because of the tendency to be unstable to both hydrolysis and oxidation.
Insufficient hydrostability of polyalkylene terephthalate can lead to chain cleavage, the extent of which can depend on the exact conditions of exposure to water or humidity. Temperature, time of exposure, and pH are all important. Both acids and bases can catalyze ester hydrolysis of polyalkylene terephthalate. Thus, if exposed to water that is acidic or basic, or if the polymer matrix involves free acidic or basic additives, decomposition can be accelerated. Since one of the reaction products of PBT hydrolysis is itself a carboxylic acid, the hydrolytic decomposition of PBT is autocatalytic.
In view of the above, there remains a need for new catalysts for the production of polyalkylene terephthalates that are effective in polymerization but that do not have potentially adverse effects in association with the polyalkylene terephthalate resin, or in blends thereof, after polymerization has been completed.